Sketch Map, S.E. Devon
General, S.E. Devon
The granite rock (red on the sketch map above, and with a thumbnail of quarried local granite picture to the left which links to a much larger image) underlying Dartmoor is of the early Permian period, around 280 million years old, and was emplaced in the roots of a mountain range then forming. However within a few miles there are rocks of greatly different ages.
Around the granite (and coloured light brown) are rocks, shales and cherts, that collected slowly in a deep sea trough during the late Devonian to early Carboniferous period, they are maybe up to 370 million years old. Later Carboniferous rocks (dark green), mostly turbidite sandstones and shales known as the Crackington and Bude Formations and deposited in shallower seas, underly mid Devon. Earlier Mid Devonian rocks, limestones and slates amongst other things, (dark brown) and close to 400 million years old, underly the north and south of Devon. Devon is thus, in simple terms, structurally a syncline. Right through the Devonian and early Carboniferous a variety of volcanic rocks are occasionally present - almost all were errupted under the sea.
All these rocks became folded and faulted in complex mountain building earth movements, known as the Variscan Orogeny, in this area at around 290 million years ago and at the end of which the granite was emplaced and mountains replaced the sea. When the hot granite was emplaced the rocks in contact with it were sometimes changed, metamorphosed, in the process.
Rocks formed after the Variscan Orogeny are mostly unfolded, and much more simple to interpret.
Underling areas just to the east of us are layers of interesting Permian rocks. The Permian was a time both of arid or desert climate and stretching of the earths crust of this area, the rocks show characteristic evidence of this. The oldest are small outcrops of early Permian Volcanic rocks, the stretching of the crust allowing lava to form and reach the surface. It was thought these lavas were never of great extent but it now seems likely that much more once existed and that it was mostly eroded away.
In the late Permian breccias, formed as alluvial fans of eroded material
carried by steeply falling but
intermittent rivers and streams flowing from
the Variscan mountains, were deposited in basins formed by the crustal
stretching. Along with the breccias there are also sandstones, some of these
were clearly desert sand dunes others show evidence of being formed from water
By the end of the Permian erosion had virtually leveled the landscape, with in
places a great depth of material
(amazingly, many Km) being deposited over
this long spell of time, the stretching crust sagged as the material was
deposited. The picture above links to various other pictures of local Permian
rocks, or else follow this link.
Later during Triassic period, and further east in Devon, finer sediments were deposited by less active water forming sandstones and mudstones. Both these rock formations and the Permian ones are pink on the map.
The sea advanced during the Jurassic with marine rocks being deposited in a shallow sea to the east of us. Rocks like these are not present in this area but some of the higher parts of the Haldon hills are composed of rocks laid down nearer to the edge of the early Cretaceous sea, about 100 million years old. The sea was deeper, and land farther away, in the later Cretaceous when huge thicknesses of the very pure limestone chalk were laid down, perhaps even over Dartmoor. Most of the chalk in Devon has now be eroded away - but at Beer there are Devon's only chalk cliffs. Finally the Bovey Basin has rocks of Tertiary period, perhaps only 30 million years old (some of which oddly also outcrop on the top of the Haldon Hills), and Dartmoor shows evidence of the effect of glaciation/s over the past million years.
A cross section shows up the geology of the Teign valley well. You can also open a window showing some of it's geology . The high plateau is made of granite, which is surrounded by metamorphosed strata, hard cherts cap or form the steeper hills, and the less steep ground shales. An interesting north south series of mineral veins also run along the west of the mid Teign valley of which more later.
The granites were intimately involved with mineralisation in the south west of England. As allready mentioned the granites were intruded into rocks - then several miles below the surface - rocks that made the roots of a mountain chain then forming. As I understand it this is, basically, what happened...
The huge pressures and very high temperatures at that depth, the slow crystallization of the rock and the liberation of water that process involved gave rise to very hot, salty, highly pressurized and chemically active 'hydrothermal' solutions that both circulated through and scavenged minerals from the setting granite mass. Heat produced by the breakdown of radioactive elements within the granite prolonged the time this circulation of hot solutions continued.
Periodically the solutions were forced out of the setting granite mass and into the surrounding rocks (with ground water from rainfall being added to the solutions and other minerals picked up) opening up new or existing fractures in the process. The minerals they carried were deposited in these fractures to form the veins and minerals that are often found in the south west. Simple.
Locally the micaceous hematite deposits of the nearby Hennock area, with similar minerals also found on our farm like the thumbnails (the top thumbnail links to a page - with large graphic files - explaining it's formation) to the right, were formed in the same way as allready mentioned.
Later, many millions of years later, rather cooler and less active solutions deposited the Barites and lead deposits also worked in several places in the mid Teign valley. The fractures these filled were probably formed in association with the opening of the Atlantic ocean. It is also relevant that, within the Carboniferous rocks of the Teign valley at least, there is evidence of the kind of 'black smoker' activity now seen at mid ocean ridges. These very hot underwater springs are well known for their mineral deposits. Some of these minerals may well have been remobilised in these later mineralisations. The Teign valley was a centre of considerable mining activity in the 19th century, mostly for these ores of Iron Lead and Barium.
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